For example, when controlling a synchronous motor that drives an electric compressor in an automobile air conditioner, periodic speed fluctuation occurs as a result of torque fluctuation due to the compressor's compression cycle. Periodic speed fluctuation causes vibrations and noise. This phenomenon is prominent in a vane compressor, and periodic load fluctuation occurs in proportion to the number of vanes during the compression process. As a result, the speed fluctuation of the motor driving the compressor ends up increasing.
Since such torque fluctuation in the compressor is periodic, methods such as a disturbance observer or repetitive control may be applied to reduce the torque fluctuation. In general, however, vibration becomes more difficult to suppress as the vibration frequency increases.
On the other hand, electric motors have been controlled by detecting the rotation position of the rotor in the motor with a sensor, yet this leads to problems such as a larger device (larger outer frame dimensions of the motor, in particular the axial length) and higher costs. Therefore, position sensorless control has recently been applied.
Position sensorless control, however, is affected by speed estimation characteristics, making suppression of vibration difficult.
The present applicant thus proposed the electric motor control device disclosed in Patent Literature 1.
The electric motor control device disclosed in Patent Literature 1 is provided with a first command current setting unit that sets a first command current of a motor to reduce a rotation speed error, a second command current setting unit that sets a second command current of the motor based on a load fluctuation frequency of the motor, a third command current setting unit that sets a third command current of the motor using the first command current and the second command current, and an inverter switching pattern generation unit that generates a drive command for the motor from at least the third command current and the actual rotation speed.
In the second command current setting unit, a peak filter having the above load fluctuation frequency as the peak frequency is inserted in parallel with a speed PI controller, the above rotation speed error is input into the peak filter, and the output of the peak filter is the second command current.
By using such a peak filter that takes the difference in speed between the target rotation speed and the estimated rotation speed as input, it is possible to suppress the periodic speed fluctuation in the vibration frequency of the motor, thereby improving the estimation accuracy of the rotation speed.